This invention generally relates to functionalized or “chain end modified” elastomeric polymers, their use in the preparation of elastomeric compositions and articles made therefrom. The invention specifically relates to the use of so called “sulfanylsilanes” as chain end modifiers for “living” anionic elastomeric polymers. The polymer “end caps” are reactive with unsaturated portions of the elastomeric polymer backbone and/or with fillers or other components present in an elastomeric composition. These modified elastomeric polymers are useful in the preparation of vulcanized elastomeric compositions having relatively low hysteresis loss. Such compositions are useful in many articles including tire treads having low rolling resistance, along with a good balance of other desirable physical and chemical properties, for example, wet skid properties, abrasion resistance, tensile strength and processability.
A major source of hysteresis in vulcanized elastomeric polymers is believed to be attributed to free polymer chain ends, that is, the section of the elastomeric polymer chain between the last cross-link and the end of the polymer chain. This free end of the polymer does not participate in any efficient elastically recoverable process, and as a result, any energy transmitted to this section of the polymer is lost. This dissipated energy leads to a pronounced hysteresis under dynamic deformation. The hysteresis loss of an elastomeric polymer composition is related to its tan δ at 60° C. value. In general, vulcanized elastomeric polymer compositions having relatively small tan δ at 60° C. values are preferred as having lower hysteresis loss. In tires, this translates to a lower rolling resistance and better fuel economy.
One generally accepted approach to reducing hysteresis loss is to reduce the number of free chain ends of elastomeric polymers. Various techniques are described in the open literature including the use of “coupling agents,” such as tin tetrachloride, which may functionalize the polymer chain end and react with unsaturated portions of the polymer backbone and/or other constituents in an elastomeric composition, such as a filler. Examples of such techniques along with other documents of interest include: U.S. Pat. Nos. 3,281,383; 3,244,664 and 3,692,874 (e.g. tetrachlorosilane); U.S. Pat. Nos. 3,978,103; 4,048,206; 4,474,908; 6,777,569 (blocked mercaptosilanes) and U.S. Pat. No. 3,078,254 (a multi-halogen-substituted hydrocarbon such as 1,3,5-tri(bromo methyl)benzene); U.S. Pat. No. 4,616,069 (tin compound and organic amino or amine compound); and U.S. 2005/0124740.
“Synthesis of end-functionalized polymer by means of living anionic polymerization,” Journal of Macromolecular Chemistry and Physics, 197, (1996), 3135-3148, describes the synthesis of polystyrene and polyisoprene containing living polymers with hydroxy (—OH) and mercapto (—SH) functional end caps obtained by reacting the living polymer with haloalkanes containing silyl ether and silyl thioether functions. The tertiary-butyldimethylsilyl (TBDMS) group is preferred as protecting group for the —OH and —SH functions in the termination reactions because the corresponding silyl ethers and thioethers are found to be both, stable and compatible with anionic living polymers.
U.S. Pat. No. 6,579,949 describes the use of similar class of sulfur compounds, including tert-butyl dimethylsilyl-3-chloro-1-propylsulfide (Cl—(CH2)3-S—Si—(CH3)2C(CH3)3) to produce rubber articles having low hysteresis loss. More specifically, the subject sulfur compounds are reacted with anionically-initiated, living polymers to produce chain end modified polymers, which are subsequently blended with fillers, vulcanizing agents, accelerators, oil extenders, and other various additives to produce tires having low hysteresis loss. The tert-butyl dimethylsilylpropylsulfide end cap is not easily removed during standard polymerization conditions, but the protective tert-dimethylsilyl group is cleaved by reaction with additives containing H+, F− or zinc compounds prior to, or during, vulcanization, thus leaving a mercapto (“thiol”) group to react (at least 20 percent) with unsaturated segments of the backbone of other elastomeric polymers. Unfortunately, the chain end modification reaction produces lithium chloride. Chloride ions present in the reaction strongly accelerate corrosion in processing equipment.
U.S. Pat. No. 6,229,036 discloses a broad class of sulfanylsilanes prepared by reacting mercaptosilanes with chlorosilanes, and their use as coupling agents in rubber mixtures to produce tire treads having low rolling resistance and good wet grip. Many sulfanylsilane compounds are described including: (EtO)3—Si—(CH2)3—S—Si—(CH3)3 and (MeO)3—Si—(CH2)3—S—Si—(C2H5)3. According to this reference, elastomeric polymers are prepared and terminated via conventional techniques, and subsequently mixed with oxidic fillers and from 0.1 to 15 weight percent (with respect to the filler) of a sulfanylsilane coupling agent, and then vulcanized to form a rubber product. Thus, unlike the approach described in U.S. Pat. No. 6,579,949, the sulfanylsilane coupling agent is not used as a chain end modifier to a living polymer, but is only combined with a post-terminated elastomeric polymer during compounding. This approach is disadvantaged due to the difficulty of distributing the coupling agent throughout the rubber mixture during compounding. That is, unlike the typical low viscous, solvent-based environment associated with most anionic polymerizations, the rubber compounding environment is typically highly viscous and solvent free, thus leading to a less homogenous distribution of the coupling agent throughout the composition. As consequence, the interaction of the functionalized polymer with the filler material and/or unsaturated segments of the polymer backbone is less complete. If the modifier compound is added to a polymer comprising exclusively terminated polymer chains, it is not possible to efficiently combine (or react) the chain ends of this polymer with other polymer chains, or with fillers, by using the modifier compound. In addition, it is not possible to efficiently combine or link the polymer to fillers or other polymer chains.